1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus including lenses.
2. Description of the Related Art
In an image pickup device such as a complementary-metal-oxide-semiconductor (CMOS) sensor or a charge-coupled-device (CCD) sensor, on-chip lenses (microlenses) are used for increasing light utilization efficiency. A plurality of on-chip lenses are arranged in correspondence with a plurality of photoelectric conversion portions, whereby a lens array is provided. Examples of such an on-chip lens include a top lens, an in-layer lens, and so forth.
Japanese Patent Laid-Open No. 2008-192951 discloses an apparatus including, as in-layer lenses, plano-convex lenses each having a convex surface on its incidence side and a planar surface on its emergence side. To suppress reflection at the interface between each in-layer lens and an optical waveguide (a second transparent film) as a medium provided on the emergence side of the in-layer lens, the ratio of the refractive index of the optical waveguide to the refractive index of the in-layer lens is set to 0.95 to 1.05.
If the refractive index of the medium provided on the emergence side of the in-layer lens is made sufficiently smaller than the refractive index of the in-layer lens, the reflectance at the emergence surface of the in-layer lens increases because of the difference in the refractive index between the in-layer lens and the medium provided on the emergence side of the in-layer lens. Hence, in a case where there is another interface having a yet different refractive index between the emergence surface and a corresponding one of photoelectric conversion portions, optical interference may occur because of the difference in the optical path length between, for example, a light beam that is transmitted through the emergence surface and the interface without being reflected by the emergence surface and the interface and a light beam that is transmitted through the emergence surface, is reflected by the interface and the emergence surface, and is transmitted through the interface. If the position (level) of the emergence surface or the interface varies among different photoelectric conversion portions, the intensity of interference also varies among the different photoelectric conversion portions. Consequently, a problem arises in that the intensity of signals obtained through photoelectric conversion varies among different photoelectric conversion portions even if light beams that enter different in-layer lenses have the same intensity. In contrast, if the difference in the refractive index between the in-layer lens and the medium on the emergence side of the in-layer lens is reduced as disclosed by Japanese Patent Laid-Open No. 2008-192951, it becomes difficult to guide light to a corresponding one of the photoelectric conversion portions, resulting in a reduction in light utilization efficiency.